1. Field of the Invention
The present invention relates to a system for protecting a thin-layer battery.
2. Discussion of the Related Art
The voltage across a thin-layer battery, as an example, of lithium-ion type, decreases along its operation to reach a critical voltage below which the battery is irreversibly damaged. The battery discharge should thus be stopped and the battery should be recharged before it reaches this critical voltage.
Thin-layer batteries used in systems with an intermittent operation, for example, batteries powering self-contained sensors which periodically communicate data (regularly or not) are here considered. In such systems, very short active periods alternate with inactive periods, which may be long. Further, the thin layer technology used for the batteries necessitates that such batteries to have a strong internal resistance with respect to other batteries.
FIG. 1 is a diagram showing, versus time t, the current I provided by a battery powering such a sensor. Current peaks of high amplitude (for example, 5 mA) and short duration, shorter than half a millisecond, are separated by long periods, lasting for from a few seconds to several hours, during which the current has a very small amplitude (for example, 0.1 mA). Between times t0 and t1, the intermittent load is inactive. The intermittent load is active between times t1 and t2 and between times t3 and t4. The presence of a non-zero current during inactive periods is for example due to the fact that, during these periods, the battery is used to power a low-consumption microcontroller.
Although systems for protecting batteries which detect the time when said batteries approach their critical voltage have been provided, as will be discussed hereafter, such systems are not adapted to thin-layer batteries with an intermittent operation.